Myopia (short-sightedness) is reaching near epidemic proportions in many parts of the world and particularly so in Asia. Myopia tends to develop in childhood and progress in severity until adulthood is attained and in some cases beyond this. Myopia carries economic disadvantages by reducing career choices and in its higher degrees carries a risk of visual impairment or blindness from such complications such as retinal detachment or myopic macular degeneration. Myopia is also associated with the development of cataract and glaucoma.
The aetiology of myopia appears to be two-fold, including genetic predisposition and exposure to as yet undefined environmental factors. Currently, the relative contributions of genetic and environmental factors to the development of childhood myopia remain a matter of dispute (Goldschmidt E., Acta Ophthalmol Scand 2003; 81:431-6; Morgan I, Rose K., Prog Retin Eye Res 2005; 24:1-38). The increasing prevalence of myopia appears to be associated with urbanisation. Previously, it was thought that factors promoting myopia were excessive close work combined with genetic predisposition (Saw S M, et al., Lancet 2001; 357 (9253):390; Mutti D O, et al., Invest Ophthalmol Vis Sci 2002; 42:3633-3640; Ip J M, et al., Invest Ophthalmol Vis Sci 2008; 49:2903-10). More recently, indoor activity unassociated with close work has been identified as a risk factor for childhood myopia (Rose K A, et al., Arch Ophthalmol 2008; 129:527-30). Outdoor activity has been shown to be protective against myopia and not just as a reciprocal of indoor activity (Rose K A, et al., Ophthalmology 2008; 115:1279-1285; Dirani M, et al., Br J Ophthalmol 2009; 93:997-1000).
Experimentally, young animals can be made myopic irrespective of their genetic makeup by modification of the visual input in early life. Thus, blurring of the visual image causes myopia in young animals (e.g. by lid closure) (Raviola E, Wiesel T N, N Engl J Med 1985; 312:1609-1615; Wallman J, et al., Science (Washington, D.C.) 1985; 201:1249-51) or the wearing of a translucent goggle (Pickett-Seltner R L, et al., Vision Res 1988; 28:323-328). This very abnormal situation however does not account for the vast majority of myopia occurring in childhood where in general, affected persons have normal spectacle corrected vision.
It has been shown that in young experimental animals myopia can be induced by the wearing of negative (−ve) spectacle lenses or contact lenses while the wearing of positive (+ve) lenses can cause hyperopia (Holden A L, et al., Eye 1988; 2 Suppl: 5242-56; Irving E L, et al., Optom Vis Sci 1991; 68:364-8; Schaeffel F, et al., Vision Res 1988; 28:639-57) even in conditions of monochromatic light (Schaeffel F, et al., Vision Res 1988; 28:639-57). For example, as shown in FIGS. 1A and 1B, the wearing of a −ve lens 100 causes the images of distant objects to be focused at a point 102 behind the retina 104 (‘hyperopic blur’), while the wearing of +ve lens 106 causes distant objects to be focused at a point 108 in front of the retina 104 (‘myopic blur’). In hyperopic blur (FIG. 1A), light traversing the retina 104 is convergent while in myopic blur (FIG. 1B), the light is divergent. The retina 104 may be able to differentiate between hyperopic and myopic blur and in the former case, to generate signals that lead to elongation of the eye and myopia.
It has been suggested that the retina uses the vergence of light traversing the retina to provide a cue to the sign (Wildsoet C F, Schmid K L. Vision Res 2001; 41:197-204) of the defocus (−ve or +ve vergence). It has also been suggested that the colour (chromatic content) of the light reaching the retina provides such cues (Seidemann A, Schaeffel F, Vision Res 2002; 42:2409-2417). There is no explanation however for the mechanisms that might allow the retina to react to the vergence of light so as to distinguish between convergent and divergent light or for the mechanisms that might allow the retina to react to chromatic cues and initiate alterations in the pattern of ocular growth and refractive development.
Attempts have been made to link chromatic-induced changes in accommodation to the etiology of myopia (Rohrer B, et al., J Physiol 1992; 449:363-376; Rucker F J, Kruger P B, Vision Res 2006; 46:3079-89; Rucker F J, Wallman J., Vision Res 2009; 49:1775-1783). There is however evidence that accommodation plays no direct etiological role in the development of myopia, for myopia can be induced by visual deprivation in experimental animals where accommodation has been surgically abolished (Troilo D. Wiley Chichester (Ciba Foundation Symposium 155) 1990; 155:89-102; discussion 102-14). It may be the case however that both accommodation and childhood myopia may be a reaction to the same stimulus, accommodation being a short term response to an out of focus image while induced myopia in experimental animals (and in humans) may be a long term response to the same features of the visual image (Wallmann J, CIBA Foundation Symposium 155, 1990 pp 1&3, John Wiley and Sons, Chichester).
Presently, one way of reducing progression of myopia is by the use of atropine eye-drops over an extended period of time (Saw S M, et al., Ophthalmology 2002; 109:415-21; Kennedy R H, et al., Binocul Vis Strabismus Q 2000; 15 (Suppl 3):281-304; Chua W H, et al., Ophthalmology 2006; 113:2285-91). Atropine has the disadvantage of dilating the pupils, causing glare and intolerance of light, and paralysis of accommodation so that sharp distance vision and near vision require either separate pairs of glasses or the use of bifocal glasses. The possible long-term adverse effects of light damage from chronically dilated pupils are not known. Myopia continues to progress when the eye-drops are discontinued.
Hyperopic overcorrection of myopia with +ve lenses may modify the rate of progression of already established myopia but such lenses while allowing a sharp image of near objects, sacrifice clarity of distance vision.
Spectacle lenses with a weaker −ve correction in the periphery than centrally have been tried with the intention of reducing the stimulus to myopia progression believed to be contributed by the relative hyperopia of the peripheral retina as compared to the central retina. So far, the effect on myopia progression has been significant but relatively small (Sankaridurg P., et al., Optom Vis Sci 2010; 87:631-41). Currently, contact lenses designed to overcome peripheral relative hyperopia are also undergoing evaluation.